Conventionally, latent images which are electrically or magnetically formed in electrophotographic image forming apparatuses are formed into visible images by means of toner for image formation (hereinafter, it may be simply referred to as “toner”). For instance, in an electrophotographic process, an electrostatic image (latent image) is formed on a surface of a photoconductor, and the latent image is developed using a toner to thereby form a toner image. The toner image is usually transferred onto a transfer material (recording medium) such as paper, and then fixed on the transfer material such as paper by heating or other method. In the step of fixing a toner image on a transfer paper, generally, thermal fixing methods, such as a heating roller fixing method and a heating belt fixing method, are widely and commonly used for their superior energy conversion efficiency.
Recently, market demands for higher-speed performance and energy saving in image forming apparatuses are more and more increasing. In response to this, a toner which is superior in low-temperature fixability and transparency and enables to provide a high-quality image is desired. In order to achieve the low-temperature fixability of toner, however, there is a need to lower the softening point of a binder resin used in the toner. When a binder resin having a low softening point is used, a part of the toner forming a toner image is attached onto a surface of a fixing member and then transferred onto sheets of copy paper, this is, so-called offset (otherwise, referred to as “hot-offset”) occurs. Furthermore, particularly under high temperature environments, the heat resistance of the toner degrades, and toner particles are fused to each other, that is, so-called blocking occurs. Besides the above-mentioned, there have been the following problems: a toner is fused on the inside of a developing device and a carrier in the developing device to cause smear; and toner filming easily occurs on a photoconductor surface. As one of measures to solve the above-mentioned problems, there has been proposed a toner which is improved in the physical properties: a toner using a polyester resin containing a polylactic acid has been proposed, which is said to be superior in storage stability, low-temperature fixability, offset resistance, environmental stability, and environmental conservation. However, the thermal properties of the polyester resin containing a polylactic acid are not sufficiently controlled as compared to polyester resins conventionally used for toner. Therefore, there are many constraints in formulation amount of the resin and production technique, and sufficient storage stability, low-temperature fixability and offset resistance have not yet obtained (see Patent Literature 1 and Patent Literature 2).
Generally, a toner used in developing of electrostatic images is composed of colored particles containing a colorant, a charge controlling agent, and the like in a binder resin. The production methods thereof are broadly classified into pulverization method and suspension polymerization method.
In the pulverization method, a colorant, a charge controlling agent, an anti-offset agent are uniformly dispersed in a thermoplastic resin to obtain a toner composition, the toner composition is pulverized and classified to thereby produce a toner. According to the pulverization method, a toner having somewhat superior physical properties can be produced, but there is a limit to selection of materials. For example, a toner composition obtainable by melt-mixing is necessarily pulverized and classified by using an economically usable device. In view of this requirement, as for a toner composition obtainable by melt-mixing, there is no choice but to make it sufficiently brittle to crush. When such a toner composition is pulverized, particles having wider particle size distribution tend to be formed. On that occasion, if an attempt is made to obtain a reproduced image with high resolution and high tone level, fine powder particles having a particle size of 5 μm or smaller and coarse powder particles having a particle size of 20 μm or greater must be removed by classification, resulting in a very poor yield. In addition, in the pulverization method, it is difficult to uniformly disperse a colorant, a charge controlling agent and the like in a thermoplastic resin, which may adversely affect the flowability, developing property, durability, image quality and the like of the resulting toner.
To solve the problems, Patent Literature 3 and Patent Literature 4 propose a dissolution suspension method using a dissolved resin, in which a resin solution in which a previously synthesized resin by polymerization reaction is dissolved is dispersed in an aqueous medium in presence of a dispersant (dispersion aid) such as a surfactant or a water-soluble resin, and a dispersion stabilizer such as resin fine particles, and the solvent is removed from the dispersion liquid by heating, reducing pressure, or the like to thereby obtain a toner. According to the dissolution suspension method, a toner having uniform particle diameter can be obtained without performing classification.
In an electrophotographic image forming appartus, in a fixing step employing a contact heating method in which a heating member such as a heating roller is used, it has been desired for toner to have releasing property (hereinafter, it may be referred to as offset resistance) to the heating member. In the dissolution suspension method using a dissolved resin, the offset resistance of toner is improved by using a modified polyester resin (see Patent Literature 5).
In the meanwhile, most of binder resins occupying 70% or more of the total amounts of toner components are derived from petroleum resources. There are concerns about exhaustion of petroleum resources and concerns that a large amount of petroleum resources is consumed and a large quantity of carbon dioxide is released into the atmosphere, leading to global-warming. Then, when resins derived from plants taking in carbon dioxide in the atmosphere to grow up are used as binder resins, carbon dioxide generated in use of the toners only circulates in the environments, and the use of plant-derived resins may make it possible to solve the global-warming problem and the problem with exhaustion of petroleum resources at a time. A variety of toners using such plant-derived resins as binder resins have been proposed. For example, Patent Literature 6 proposes to use a polylactic acid as a binder resin. However, when a polylactic acid is directly used as a binder resin according to the proposal, the concentration of ester linkage of the binder resin is higher than that of a polyester resin, and thus, the effect as a thermoplastic resin becomes weak in fixing step of toner image. Moreover, the toner becomes very hard, laking in pulverizability, and resulting in degradation of productivity.
An electrostatic image developing toner is proposed in Patent Literature 7, which contains a polyester resin obtained by dehydration polycondensation of a composition containing a lactic acid, and a trifunctional or higher-functional oxycarboxylic acid, and a colorant. However, in this proposal, the polyester resin is formed by a dehydration polycondensation reaction between a hydroxyl group of lactic acid and a carboxyl group of oxycarboxylic acid, and thus the molecular weight is increased to impair the sharp-melt property and low-temperature fixability.
In order to improve thermal properties of toner, Patent Literature 8 discloses an electrophotographic toner containing a polylactic acid-based biodegradable resin and a terpene-phenol copolymer, which however, cannot satisfy both the low-temperature fixability and the hot-offset property simultaneously.
Since the toners relating to the prior art are obtainable by a pulverization method, it involves problems of toner loss caused by classification, and toner waste accompanied therewith. In addition, because the energy quantity required for pulverization method is relatively large, it is necessary to further reduce environmental load.
Polylactic acids, which are generally used and easily available, are synthesized by dehydration condensation of a lactic acid, as described in Patent Literature 9 and Patent Literature 10, or by ring-opening polymerization of a cyclic lactide of lactic acid. For this reason, when a toner is produced using a polylactic acid, the dissolution suspension method using a dissolved resin, as disclosed in Patent Literature 3 to Patent Literature 5 can be used. However, since a polylactic acid having only L body or D body has high crystallinity, the solubility in organic solvents is extremely low, and thus it is difficult to use dissolution suspension method using dissolved resin. Then, the solubility of lactic acid in organic solvents can be improved by mixing L body of a polylactic acid and D body of a polylactic acid to decrease the crystallinity.
In the meanwhile, since polylactic acids are difficult to control their molecular weights, and ester linkages are present via only carbon atoms, it is difficult to impart necessary physical properties to toner by using polylactic acid along. In contrast, as used in conventional methods, it can be considered to provide necessary physical properties and thermal properties to toner by using a mixture of a polylactic acid and other resin or resins. However, polylactic acids are extremely poor in solubility and dispersibility in polyester resins and styrene-acryl copolymers which are generally used for toner, and thus it is very difficult to produce a toner in such a manner.
Furthermore, since the rate of crystallization of polylactic acids is rather slow, a toner produced by dissolution suspension method using a dissolved resin is difficult to control the crystallized state of polylactic acid, and in a toner produced by the method, a polylactic acid having high-crystallinity and a polylactic acid having low-crystallinity are present in a mixed manner. Therefore, portions having the high-crystalline polylactic acid are grown into crystals with a lapse of time, causing changes in charged amount and image density of the resulting toner as time goes by.
Accordingly, a toner which are superior in image density, fixability, and heat-resistant storage stability, causes less changes in fixability with a lapse of time and which contains a polylactic acid, and the related techniques have not yet been obtained, and further improvements and developments are still desired.